1. Field of the Invention
This invention relates to sulfonated conjugate fibers having a surface portion constituted by a specific ethylene copolymer and a core portion constituted by polypropylene, a non-woven fabric sheet obtainable from such conjugate fibers, a storage battery separator using such non-woven fabric sheet and a method of manufacturing such battery separator.
2. Description of the Prior Art
Polyolefin fibers are chemically stable fibers and extensively utilized as non-woven fabric sheets and the like. When forming a non-woven fabric sheet, integrity is improved usually with the use of low melting point components having excellent thermal bonding properties. More specifically, core-sheath conjugate fibers are used, which consist of a core portion constituted by polypropylene and a sheath portion constituted by polyethylene or like low melting point compounds. Such conjugate fibers, however, are usually poorly hydrophilic, thus posing limitations on their use as storage battery separators and like purposes.
For example, as an alkali battery separator there have been proposed a polyolefin non-woven fabric sheet obtained by carrying out a surface active agent treatment to enhance the hydrophilicity and a non-woven fabric sheet mainly consisting of polyamide fibers having superior hydrophilicity although the oxidization resistance is inferior, these examples of a non-woven fabric sheet being disclosed in Japanese unexamined patent application laid open (Tokkai) No. 147956/1983, 194255/1983 and 39362/1986.
Non-woven fabric sheets used for alkali storage battery separators are required to have resistance against alkali, i.e., strongly alkaline electrolytes, and also resistance against oxidization, i.e., electrolytic oxygen generated during charging. In addition, it has to have a high specific volume sufficient to maintain permeation of and wetting by electrolysis. Further, it has to have sufficient shape stability to be free from interelectrode short-circuit that results from dielectric breakdown stemming from denaturing, shrinkage or dissolution caused during use and to also have adequate mechanical strength for assembling operations.
Polyamide fibers which are extensively used up to date for non-woven fabric sheet for storage battery separators have neither sufficient alkali resistance nor sufficient oxidization resistance, and also their shape stability is unsatisfactory.
Accordingly, there have been attempts to subject polyolefin fibers to surface active agent treatment before use. In this case, however, with charging and discharging carried out a number of times the surface active agent is reduced. This means that the hydrophilicity is gradually deteriorated.
To solve the above problems, it has been intended to use ethylene-vinylalcohol copolymer fibers as disclosed in Japanese unexamined patent application laid open (Tokkai) No. 34849/1988 or use polyethylene or polypropylene fibers after sulfonization as disclosed in Japanese unexamined patent application laid open (Tokkai) No. 175256/1983, 57568/1989, 132043/1989 and 132044/1989 and EpC 0316916 A2 (1989/05/24).
According to those prior arts as noted above, however, polyolefin is subjected to graft-copolymerization with acrylic acid or the like using radioactive rays, and the resultant copolymer is sulfonated with fuming sulfuric acid or the like. This means that hydrogen substitution reaction of tertiary carbon is involved. The reaction efficiency of this reaction is inferior and poses problems in the process of manufacture. In addition, introduction of sulfo groups can not be effected efficiently, and it is difficult to control the reaction factor.
Further, according to the EPC Disclosure 0316916A2 as noted above acrylic acid copolymerized polyethylene is pulverized after sulfonization, and resultant particles are bonded by adhesive to the surface of polypropylene fibers, thus presenting the problem of ready detachment of the bonded particles from the surface.